Spindle motor

ABSTRACT

A potential-difference alleviating member for alleviating and lowering the potential difference, which is an energy difference between a rotating or fixed bearing member and a rotary hub or a fixing frame which are formed of metals of different types, is interposed between the two members so as to prevent the occurrence or advance of potential difference corrosion. Relief portions are respectively provided at a joining interface between a rotary shaft and a thrust plate and a joining interface between a bearing member and the counter plate, and the respective members are welded in the relief portions so as to be integrated.

BACKGROUND OF THE INVENTION

The present invention relates to a spindle motor used as an apparatusfor rotatively driving a hard disk or the like.

A spindle motor disclosed in, for example, Japanese Patent PublicationNo. 8-4769A is known as a spindle motor used as an apparatus forrotatively driving a recording medium such as a hard disk. As shown inFIG. 7, this spindle motor is mainly comprised of a stator assembly 100and a rotor assembly 120 having driving magnets 125. The rotor assembly120 has a hub 122 secured to an upper end portion of a rotary shaft 121by means of press-fitting, shrinkage fitting, or the like. Meanwhile,the stator assembly 100 has stator cores 116 each formed by winding acoil 117 around a respective salient pole portion. These stator cores116 are fitted to an outer peripheral portion of a bearing holder 115.

A bearing sleeve 113 is fitted in an inner peripheral portion of thebearing holder 115. Radial bearing portions RBa and RBb serving asbearing surfaces for generating hydrodynamic pressure are formed on aninner peripheral surface of the bearing sleeve 113 in such a manner asto be spaced apart from each other in the axial direction. A lubricatingfluid 105 such as oil undergoes a pressure rise due to the pumpingaction of dynamic pressure generating grooves (not shown) when therotary shaft 121 rotates, and the rotary shaft 121 and the hub 122 arepivotally supported by the hydrodynamic pressure generated by thelubricating fluid 105.

Further, a thrust plate 126 constituting a thrust hydrodynamic bearingportion is press-fitted and secured to the rotary shaft 121. Further, acounter plate 114 is fixed at an open end of the bearing holder 115 of aframe 111 through a mechanical coupling means such as fixing screws 106.The thrust plate 126 is placed between a lower end face of the bearingsleeve 113 and an inner bottom surface of the counter plate 114, and asthe lubricating fluid 105 is present in this space, the rotary shaft 121is stably supported in the thrust direction by the hydrodynamic pressuregenerated by the lubricating fluid 105.

In recent years, a trend toward compact and thin spindle motors forrotatively driving recording medium disks are rapidly underway. Inconjunction with this trend, the bearing member (bearing sleeve 113)supporting the shaft 121 is formed of a metallic material different fromthe metallic material composing the fixing frame 111. One reason forthis is that a metal excelling in workability is adopted as the metallicmaterial composing the bearing sleeve 113 so that the inside-diameterportion of the bearing sleeve 113 can be machined satisfactorily. Inthis case, the bearing sleeve 113 formed of a different type of metallicmaterial is integrally joined to the fixing frame 111 by means ofpress-fitting, shrinkage fitting, or the like.

In a spindle motor in which different types of metallic material areintegrally joined together, if an electrolyte having a large dielectricconstant, such as water, penetrates the joint, a local battery is formedbetween these metallic materials of different types, and anodicdissolution occurs due to the local battery, resulting in the so-calledpotential difference corrosion. The portion where such potentialdifference corrosion occurs is scattered in due course of time in theform of dust, and causes damage to the recording medium disk or themagnetic head. Accordingly, in the case of an apparatus for whichcleanliness is required, such as a hard disk drive (HDD), it isdesirable to reliably prevent the occurrence of the aforementionedpotential difference corrosion.

In recent years when motors are required to be thinner, it has becomeimpossible to secure a sufficient joining length in the joining of therotary shaft and the thrust plate and in the joining of the rotary shaftand the hub. Consequently, there have arisen problems in that it isdifficult to obtain desired shock-resisting performance (e.g., 1,0000 Gor more) and joining strength capable of withstanding an external stressduring assembly, thereby making it difficult to produce a thin motor.

For instance, in FIG. 7, various joining methods are adopted in joiningthe counter plate 114 and the frame 111 or in joining the counter plate114 and the bearing sleeve 113. In a case where the fixing screws 106shown in FIG. 7 are used to effect fastening, the heads of the fixingscrews 106 hinder the attempt to produce a thin motor. In a case wherethe counter plate 114 is fixed by a calking method, the calked portionmust be made to project from the bottom surface of the counter plate114, which also hinders the attempt to produce a thin motor. Further, ina case where the counter plate 114 is fixed by a press-fitting method,since a sufficient joining length cannot be obtained, the joiningstrength lacks.

SUMMARY OF THE INVENTION

A primary object of the invention is to provide a spindle motor whichmakes it possible to prevent by a simple arrangement the potentialdifference corrosion between a bearing member and another member whichare formed of metallic materials of different types.

A secondary object of the invention is to provide a spindle motor whichcan be made thin by increasing the joining strength even in the case ofa part whose joining length is short.

In accordance with the invention, the arrangement is provided such thata potential-difference alleviating member for alleviating and loweringthe potential difference, which is an energy difference between arotating or fixed bearing member and a rotary hub or a fixing framewhich are formed of metals of different types, is interposed between thetwo members so as to prevent the occurrence or advance of potentialdifference corrosion. Accordingly, the working environment of anapparatus such as a hard disk drive (HDD), in particular, for whichcleanliness is required, can be made favorable, and the reliability ofthe apparatus can be improved.

Further, in accordance with the invention, the arrangement is providedsuch that an insulating resin coating film or a passivation film isinterposed between a rotating or fixed bearing member and a rotary hubor a fixing frame which are formed of metals of different types, so asto prevent the occurrence of a local battery and prevent the occurrenceor advance of potential difference corrosion. Accordingly, the workingenvironment of an apparatus such as a hard disk drive (HDD), inparticular, for which cleanliness is required, can be made favorable,and the reliability of the apparatus can be improved.

Furthermore, in accordance with the invention, the arrangement isprovided such that relief portions are respectively provided at ajoining interface between the rotary shaft and the thrust plate and ajoining interface between the bearing member and the counter plate or ajoining interface between the fixing frame and the counter plate, andthe respective members are welded in the relief portions so as to beintegrated. Accordingly, even if the joining length of the members isrelatively short, it is possible to obtain a sufficient joining strengthand improve the shock resistance of the motor itself. As a result, theperpendicularity of the thrust plate with respect to the rotary shaft,for example, can be maintained stably, and the reliability of the motorcan be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is an explanatory cross-sectional view showing a hard-diskdriving motor of a shaft fixed type according to a first embodiment ofthe present invention;

FIG. 2 is an explanatory half cross-sectional view showing a hard-diskdriving motor of a shaft rotating type according to a second embodimentof the present invention;

FIG. 3 is an explanatory cross-sectional view showing a hard-diskdriving motor of a shaft fixed type according to a third embodiment ofthe present invention;

FIGS. 4A to 4C are cross-sectional views showing the structure forjoining a rotary shaft and a thrust plate;

FIG. 5 is a half cross-sectional view showing a spindle motor accordingto a fourth embodiment of the present invention;

FIG. 6 is a cross-sectional view showing the structure for joining thefixed shaft and the thrust plate shown in FIG. 1; and

FIG. 7 is a half cross-sectional view showing a related spindle motor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereafter, a description will be given of the embodiments of theinvention. First, referring to the drawings, a description will be givenof the structure of a hard disk drive (HDD) to which the invention isapplied.

The HDD spindle motor of a shaft fixed type, which is a first embodimentof the present invention, shown in FIG. 1 is comprised of a statorassembly 10 serving as a fixed member and a rotor assembly 20 serving asa rotating member which is assembled to the stator assembly 10 from anupper side thereof in the drawing. Of these assemblies, the statorassembly 10 has a fixing frame 11 which is screwed down to anunillustrated fixed base. A hollow cylindrical bearing holder 12 isformed on a substantially central portion of the fixing frame 11 in sucha manner as to be integrally provided uprightly, and stator cores 14 arefitted to an outer peripheral surface of the bearing holder 12. Drivingcoils 15 are respectively wound around salient pole portions of thestator cores 14.

A fixed shaft 16 formed of a stainless steel (SUS 420J2; indicationbased on JIS) is fixed in a shaft-fixing hole 11 a of the fixing frame11 in such a manner as to project upwardly. This fixed shaft 16 isdisposed concentrically with the bearing holder 12, and an upper endportion of the fixed shaft 16 is also screwed down to the unillustratedfixed base. A bearing sleeve 21 serving as a rotating-shaft bearingmember making up a part of the rotor assembly 20 is rotatably fitted onan outer periphery of the fixed shaft 16, and a rotary hub 22 formounting an unillustrated recording medium such as a magnetic disk isjoined to an outer periphery of the bearing sleeve 21.

A cylindrical large-diameter portion 20 a for joining, which is formedin such a manner as to project outwardly in the radial direction, isdisposed in an upper end portion of the bearing sleeve 21. A joininghole 22 a, which is formed penetratingly in a central portion of therotary hub 22, is integrally joined to an outer peripheral surface ofthe large-diameter portion 20 a for joining by means of press-fitting orshrinkage fitting. The rotary hub 22 is formed of an aluminum groupmaterial for the purpose of light weight, and has a cylindrical body 22e. Annular driving magnets 22 c are attached to an outer periphery ofthe cylindrical body 22 e with a back yoke 22 b placed therebetween.These magnets 22 c are disposed in such a manner as to annularly opposeouter peripheral-side end faces of the stator cores 14 in closeproximity thereto. Further, the cylindrical body 22 e has adisk-mounting surface 22 d for mounting the recording medium disk on itsouter peripheral portion.

Meanwhile, the bearing sleeve 21 is formed of a copper group material ora stainless steel metal to facilitate drilling and the like. A pair ofbearing projections 21 a serving as a pair of radial bearings are formedon an inner periphery of a central hole, which is provided in thebearing sleeve 21, in such a manner as to be axially spaced apart apredetermined distance. Further, hydrodynamic surfaces formed on innerperipheral surfaces of these bearing projections 21 a are disposed insuch a manner as to proximately oppose hydrodynamic surfaces formed onan outer peripheral surface of the fixed shaft 16, thereby forming apair of radial hydrodynamic bearing portions RBa and RBb which areadjacent to each other in the axial direction. More specifically, thehydrodynamic surface on the bearing sleeve 21 side and the hydrodynamicsurface on the fixed shaft 16 side in each of the pair of radialhydrodynamic bearing portions RBa and RBb are opposingly disposedcircumferentially with a very small gap of several microns therebetween.A lubricating fluid such as oil, a magnetic fluid, or air is charged inthe bearing space having the very small gap in such a manner as tocontinue in the axial direction. It should be noted that oil is used asthe lubricating fluid in this embodiment.

A fluid storage portion 21 b, which is formed by radially recessing theinner peripheral portion of the central hole in the bearing sleeve 21,is formed between the pair of radial hydrodynamic bearing portions RBaand RBb, and a sufficient quantity of lubricating fluid is stored in thefluid storage portion 21 b.

At least one of the hydrodynamic surfaces of the bearing sleeve 21 andthe fixed shaft 16 is annually recessed to form unillustrated radialdynamic pressure generating grooves of, for example, herringbone shapein such a manner as to be axially divided into two blocks. Thus thelubricating fluid is pressurized by the pumping action of the radialdynamic pressure generating grooves during rotation to generatehydrodynamic pressure, and the rotary hub 22 is pivotally supported inthe radial direction by the hydrodynamic pressure of the lubricatingfluid.

In the axially opposite end portions of the bearing space which form theradial hydrodynamic bearing portions RBa and RBb, a pair of capillaryseal portions are respectively disposed in such a manner as to axiallysandwich the radial hydrodynamic bearing portions RBa and RBb. Each ofthese capillary seal portions is formed by gradually enlarging the gapbetween the bearing sleeve 21 and the fixed shaft 16 in the radiallyoutward direction in a tapered manner by an inclined surface formed onthe bearing sleeve 21. The dimension of the gap of the capillary sealportion disposed on the inward side of the bearing is set to a range of20 μm to 300 μm, for example. These capillary seal portions are soarranged that the level of the lubricating fluid is located there whenthe motor either rotates or is at a standstill.

A disk-shaped thrust plate 23 is secured to an illustrated upper endportion of the fixed shaft 16. This thrust plate 23 is disposed so as tobe accommodated in a hollow cylindrical recessed portion formed in acentral portion of the upper end of the bearing sleeve 21. Axiallyproximately opposing surfaces of the thrust plate 23 and the bearingsleeve 21 in the recessed portion of the bearing sleeve 21 are formed ashydrodynamic surfaces, thereby forming a lower thrust hydrodynamicbearing portion SBa.

Further, a counter plate 24 formed of a large disk-shaped member issecured to the upper end portion of the bearing sleeve 21 so as to belocated in close proximity to the illustrated upper hydrodynamic surfaceof the thrust plate 23. An upper thrust hydrodynamic bearing portion SBais formed by the hydrodynamic surface provided on the lower surface ofthe counter plate 24 and the hydrodynamic surface of the thrust plate 23side.

Both hydrodynamic surfaces on the thrust plate 23 side in the pair ofthrust hydrodynamic bearing portions SBa and SBb which are disposedaxially adjacent to each other, and both hydrodynamic surfaces on thebearing sleeve 21 and the counter plate 24 side which are respectivelyopposed thereto, are disposed in face-to-face relation to each other inthe axial direction with very small gaps of several micronstherebetween. The lubricating fluid such as oil, a magnetic fluid, orair is charged in the bearing spaces having the very small gaps in sucha manner as to continue in the axial direction through outerperipheral-side passages in the thrust plate 23.

Further, at least one of the hydrodynamic surfaces of the thrust plate23 on the one hand, and the hydrodynamic surfaces of the bearing sleeve21 and the fixed shaft 16 on the other hand, is annually recessed toform unillustrated thrust dynamic pressure generating grooves of, forexample, herringbone shape in such a manner as to be radially dividedinto two blocks. Thus the lubricating fluid is pressurized by thepumping action of the thrust dynamic pressure generating grooves duringrotation to generate hydrodynamic pressure, and the rotary hub 22 ispivotally supported in the thrust direction by the hydrodynamic pressureof the lubricating fluid.

Next, a description will be given of the technique of the invention forpreventing the potential difference corrosion which can occur betweenthe bearing member (bearing sleeve) and another member which are formedof metallic materials of different types.

As described above, the bearing sleeve 21 is formed of a copper groupmaterial, e.g., phosphor bronze which is one of copper alloys, while therotary hub 22 which is integrally joined to the bearing sleeve 21 isformed of an aluminum group material, e.g., an aluminum material. Thesemetals of different types are joined, thereby forming a one-piecerotating member. A potential-difference alleviating member A isinterposed between the joined surfaces of the bearing sleeve 21 and therotary hub 22. This potential-difference alleviating member A is formedof a metallic material, such as a nickel material, whose ionizationtendency in the electrochemical series with respect to solution, e.g.,plain water (tap water), is positioned between that of copper and thatof aluminum. This potential-difference alleviating member A is formed onat least one of the joined surfaces of the bearing sleeve 21 and therotary hub 22 in film form by plating processing, vapor depositionprocessing, or coating.

The ionization tendency refers to the tendency whereby a metal producescations when coming into contact with a liquid, particularly water, andcan be quantitatively evaluated by the standard electrode potential ofthe metal. The list of metals in which their ionization tendencies withrespect to solution are arranged in the order of their magnitude isreferred to as the electrochemical series.

In a case where the metallic materials of different types are copper andaluminum, metallic materials whose ionization tendencies in theelectrochemical series with respect to plain water are positionedbetween that of copper and that of aluminum are Co, Mo, Cr, and Ni,while metallic materials whose ionization tendencies in theelectrochemical series with respect to saline water are positionedtherebetween are Fe, Sn, Co, W, Cr, Mo, and Ni. The ionizationtendencies in the electrochemical series with respect to solution typesare thus known. Accordingly, the material which is used as thepotential-difference alleviating member A is selected on the basis ofthe aqueous solution which is considered to attach to the metals as wellas the two metallic materials to be joined.

In the above-described embodiment, supposing that thepotential-difference alleviating member A is not provided, since therotating member is used in which the bearing sleeve 21 formed of acopper material and the rotary hub 22 formed of an aluminum material arejoined, if an electrolyte having a large dielectric constant, such aswater, penetrates the joint, a local battery is formed between themetallic materials of different types. Hence, anodic dissolution canpossibly occur due to the local battery, resulting in potentialdifference corrosion. In contrast, in accordance with the invention,since the nickel film is provided as the potential-differencealleviating member A between the bearing sleeve 21 and the rotary hub22, the potential difference between the two members 21 and 22 becomessmall due to the potential-difference alleviating member A interposedbetween the two members 21 and 22, thereby making it possible to preventthe generation of the local battery and hamper the occurrence or advanceof the potential difference corrosion. This action of preventing thepotential difference corrosion is effective when one component part isformed by joining different types of metals as in the case of thebearing sleeve 21 and the rotary hub 22, and an energy difference(potential difference) occurs between the joined members.

As described above, the metal whose ionization tendency in theelectrochemical series with respect to solution is positioned betweenthose of two metals to be joined, i.e., the potential-differencealleviating member A, can be selected from among a number of materials.Hence, it suffices to select a material to be formed by taking intoconsideration a desired manufacturing method such as plating processing,vapor deposition processing, or coating. If the material is selectedfrom this perspective, by merely adding such as a plating process tonormal machining and assembling processes, it becomes possible to easilyprovide the potential-difference alleviating member A having asatisfactory function.

Although, in the first embodiment, a description has been given of acase in which a component part formed by joining metals of differenttypes is formed by a copper material including a copper alloy and analuminum material including an aluminum alloy, the selection of thesemetals of different types may be changed as required.

Meanwhile, the invention is similarly applicable to a spindle motor of ashaft rotating type whose half cross sectional view is shown in FIG. 2,which is a second embodiment of the present invention.

The overall HDD spindle motor of the shaft rotating type shown in FIG. 2is comprised of a stator assembly 30 serving as a fixed member and arotor assembly 40 serving as a rotating member assembled to the statorassembly 30 from an upper side thereof in the drawing. Of theseassemblies, the stator assembly 30 has a fixing frame 31 which isscrewed down to an unillustrated fixed base. The fixing frame 31 isformed of an aluminum group material to attain light weight. A bearingsleeve 33 serving as a fixed bearing member formed in a hollowcylindrical shape is integrally joined to the inner side of an annularmounting portion 32, which is formed in such a manner as to be provideduprightly in a substantially central portion of the fixing frame 31, bypress-fitting or shrinkage fitting.

The lower outer peripheral surface of the bearing sleeve 33 is formedsuch that its radial dimension substantially coincides with the radialdimension of the outer peripheral surface of the annular mountingportion 32. Stator cores 34 are fitted to an attaching surface formed byan outer peripheral surface of the bearing sleeve 33. Driving coils 35are respectively wound around salient pole portions provided in thestator cores 34. In the embodiment shown FIG. 2, although the statorcores 34 are fitted to the attaching surface formed by the outerperipheral surface of the bearing sleeve 33, an arrangement may beprovided such that the annular mounting portion 32 is extended upwardly,and the stator cores 34 are attached to an outer peripheral surface ofthat annular mounting portion 32.

A rotary shaft 41 formed of a stainless steel (SUS 420J2) or the likeand making up a part of the rotor assembly 40 is rotatably inserted in acentral hole provided in the bearing sleeve 33. Namely, hydrodynamicsurfaces formed on the inner peripheral surface of the bearing sleeve 33are disposed in such a manner as to proximately oppose hydrodynamicsurfaces formed on the outer peripheral surface of the rotary shaft 41,thereby forming the pair of radial hydrodynamic bearing portions RBa andRBb which are adjacent to each other in the axial direction. Thehydrodynamic surface on the bearing sleeve 33 side and the hydrodynamicsurface on the rotary shaft 41 side in each of the pair of radialhydrodynamic bearing portions RBa and RBb are opposingly disposedcircumferentially with a very small gap of several microns therebetween.A lubricating fluid such as oil, a magnetic fluid, or air can be used inthe bearing space.

The bearing sleeve 33 is formed of a copper group material or astainless steel to facilitate machining, and radial dynamic pressuregenerating grooves of, for example, herringbone shape are formed in itsinner periphery in such a manner as to be axially divided into twoblocks. Thus a rotary hub 42 together with the rotary shaft 41 ispivotally supported in the radial direction by the hydrodynamic pressureof the lubricating fluid during rotation.

The substantially cup-shaped hub 42 on which a recording medium such asa magnetic disk is mounted is secured to one end of the rotary shaft 41by means of a joining member which will be described later. The hub 42has a hollow cylindrical portion 42 a to which the disk is fitted, aswell as a disk mounting surface 42 b which expands outwardly from thelower end of the hollow cylindrical portion 42 a for mounting the diskthereon. Annular driving magnets 25 having magnetized poles are fittedto an inner peripheral surface of the hollow cylindrical portion 42 a ofthe hub 42, and inner peripheral surfaces of the driving magnets 25 areopposed to outer peripheral surfaces of the stator cores 34 with anappropriate interval therebetween. Here, since the hub 42 is formed of amagnetic material such as iron, the hub 42 itself can be made tofunction as a back yoke for the driving magnets 25. Accordingly, in thisembodiment, since the yoke which is a separate component is omitted, ascompared with a hub 42 having an identical outside diameter and theyoke, the inner space of the hub 42, i.e., the space for disposing thearmature, can be made large. Accordingly, it is possible to obtain arelatively large motor torque. It should be noted that in a case wherethe hub 42 is formed of a nonmagnetic material such as an aluminumalloy, a yoke formed of a magnetic material is interposed between thehub 42 and the driving magnets 25.

Meanwhile, a disk-shaped thrust plate 43 is secured to the other endside, i.e., on the lower side in the drawing, of the rotary shaft 41 bymeans of a joining member which will be described later. This thrustplate 43 is disposed so as to be accommodated in a recessed portion 33 aformed in a central portion of the lower end side of the bearing sleeve33. The upper thrust hydrodynamic bearing portion SBa is formed byhydrodynamic surfaces formed by axially proximately opposing end facesof the thrust plate 43 and the bearing sleeve 33 in the recessed portion33 a of the bearing sleeve 33.

Further, a disk-shaped counter plate 44 larger than the thrust plate 43is secured in a lower end-side opening of the bearing sleeve 33 by ajoining member, which will be described later, in such a manner as to belocated in close proximity to the illustrated upper hydrodynamic surfaceof the thrust plate 43. Then, the lower thrust hydrodynamic bearingportion SBb is formed by the hydrodynamic surface provided on an upperend face of the counter plate 44 and the hydrodynamic surface on thethrust plate 44 side.

The hydrodynamic surfaces on the thrust plate 43 side in the pair ofthrust hydrodynamic bearing portions SBa and SBb which are disposedaxially adjacent to each other, and the hydrodynamic surfaces on thebearing sleeve 33 and the counter plate 44 side which are respectivelyopposed thereto, are disposed in face-to-face relation to each other inthe axial direction with very small gaps of several micronstherebetween. A lubricating fluid 5 is charged in the bearing spaceshaving the very small gaps in such a manner as to continue in the entireaxial direction through outer peripheral-side passages in the thrustplate 43.

At least one of the hydrodynamic surfaces of the thrust plate 43 on theone hand, and the hydrodynamic surfaces of the bearing sleeve 33 and thecounter plate 44 on the other hand, is annually recessed in the usualmanner to form thrust dynamic pressure generating grooves of herringboneshape or spiral shape. Thus, when the thrust plate 43 is rotated inconjunction with the rotation of the rotor assembly 40, the rotorassembly 40 including the rotary shaft 41 and the hub 42 is pivotallysupported in the thrust direction by the hydrodynamic pressure of thethrust dynamic pressure generating grooves.

As described above, the bearing sleeve 33 is formed of a copper groupmaterial, specifically phosphor bronze, to facilitate machining, whilethe fixing frame 31 which is integrally joined to the bearing sleeve 33is formed of an aluminum group material, specifically an aluminummaterial. These metals of different types are joined. Apotential-difference alleviating member B is interposed between thejoined surfaces of the bearing sleeve 33 and the fixing frame 31. In thesame way as in the already-described embodiment shown in FIG. 1, thispotential-difference alleviating member B is formed of a metallicmaterial, such as a nickel material, whose ionization tendency in theelectrochemical series is positioned between that of a copper groupmaterial and that of an aluminum group material. Thispotential-difference alleviating member B can be formed by being coatedon at least one of the joined surfaces of the bearing sleeve 33 and thefixing frame 31 in film form by plating processing, vapor depositionprocessing, or coating.

It should be noted that the potential-difference alleviating member Bmay be formed by a passivation film B coated on at least one of thejoined surfaces of the bearing sleeve 33 and the fixing frame 31.

This passivation film B is an oxide film excelling in corrosionresistance, and can be obtained by subjecting the joined surface of thebearing sleeve 33 or the fixing frame 31 to electroless nickel-phosphorplating and by oxidizing and giving passivity to the plated film bybeing left to stand for a predetermined duration.

It should be note that, as for the passivation film B, the metallicmaterial itself forming the bearing sleeve 33 or the fixing frame 31 maybe used as the passivation film instead of using a plating materialdifferent from the metal to be joined. For example, an alumite film maybe formed on the joined surface of the fixing frame 31 which is formedof an aluminum material and is joined to the bearing sleeve 33, and itis possible to prevent the formation of a local battery in the eventthat an electrolyte having a large dielectric constant, such as water,has penetrated the joined portions of the fixing frame 31 and thebearing sleeve 33.

In this embodiment as well, the energy difference between the bearingsleeve 33 and the fixing frame 31 in which metals of different types arejoined, i.e., the potential difference between the two members 33 and31, can be alleviated and lowered by the potential-differencealleviating member B interposed between the two members 33 and 31,thereby making it possible to prevent the occurrence or advance ofpotential difference corrosion.

Next, in a third embodiment shown in FIG. 3, instead of thepotential-difference alleviating member A in the first embodiment shownin FIG. 1, an insulating resin coating film C is interposed between thejoined surfaces of the bearing sleeve 21 and the rotary hub 22. Thisresin coating film C is continuously formed over the entirecircumferential periphery ranging from the inner joined portions of thebearing sleeve 21 and the rotary hub 22, to which water or the like isliable to be attached, to outer exposed surfaces of the bearing sleeve21 on the upper and lower sides thereof in the drawing. In the innerjoined portions of the bearing sleeve 21 and the rotary hub 22, a regionis provided where the resin coating film C is not formed and is left ina notched state, so that the bearing sleeve 21 and the rotary hub 22 aremade electrically conductive. Accordingly, the joined surfaces of thebearing sleeve 21 and the rotary hub 22 are made electrically conductiveat the notched portion of the resin coating film C.

In the embodiment having the above-described configuration, since thebearing sleeve 21 and the rotary hub 22 formed of metals of differenttypes are electrically insulated by the resin coating film C, even ifwaterdrops are attached, the local battery is not formed. Consequently,it is possible to prevent the occurrence or advance of potentialdifference corrosion. Since the attachment of the waterdrops cause aproblem in the joined portions exposed to the outside, in thisembodiment in which the resin coating film C is continuously formed upto the outer exposed surfaces of the bearing sleeve 21 extendingcontinuously at the joined surfaces, the formation of the local batterycan be prevented satisfactorily even if the electrolyte such as water isattached to the outer exposed surfaces of the joined surfaces.

The arrangement in which the potential-difference alleviating member orthe passivation film is formed over the entire periphery up to the outerexposed surfaces of the bearing sleeve 21 extending continuously at thejoined surfaces can be also applied to the embodiments alreadydescribed.

In this embodiment, since the bearing sleeve 21 and the rotary hub 22are electrically insulated by the resin coating film C, while insidepart of the joined surfaces is made electrically conductive without theresin coating C, an arrangement can be provided to ground the rotary hub22 through that conductive portion. Accordingly, even if staticelectricity has been generated in the rotary hub 22, discharging can beeffected smoothly, so that damage or the like to the magnetic head dueto the static electricity can be prevented.

Such a resin coating film C is similarly applicable to the spindle motorof the shaft rotating type shown in FIG. 2. If a similar resin coatingfilm C is formed between the bearing sleeve 33 and the fixing frame 31,it is possible to obtain similar effect and advantages.

It should be noted that the invention can be similarly applied to anyportion if it is a portion where metals of different types are joined.For example, in the embodiment shown in FIG. 2, a potential-differencealleviating member may be interposed between the joining portions of therotary shaft 41 and the rotary hub 42.

Next, a description will be given of the technique of the invention forenhancing the joining strength of component parts even if joining lengthis small.

FIGS. 4A to 4C are diagrams explaining the structure for joining therotary shaft 41 and the thrust plate 43 of the spindle motor inaccordance with the second embodiment.

If the spindle motor is made thin and is designed to a height of, forexample, 5 mm or thereabouts, the joining length of the rotary shaft 41and the thrust plate 43 becomes less than 1 mm. Accordingly, the joiningstrength becomes weak since a sufficient joining length cannot beobtained even if the joining of the two members is effected by thepress-fitting method or the shrinkage fitting method. If press-fittingis effected by providing a large press-fitting allowance, there is apossibility of deterioration of the perpendicularity of the thrust plate43 with respect to the rotary shaft 41, so that a press-fittingallowance of more than a predetermined amount cannot be provided.Accordingly, in this embodiment, after the rotary shaft 41 and thethrust plate 43 are press-fitted or inserted by an appropriatepress-fitting force to such an extent that the deterioration ofperpendicularity does not occur, the joining interface portions of thetwo members are welded together. At this juncture, an axially recessedrelief portion 70 is annularly formed in advance at the surface portionof the joining interface portion, and the rotary shaft 41 and the thrustplate 43 are welded in this relief portion 70.

The shape of the relief portion 70 at the joining interface between therotary shaft 41 and the thrust plate 43 is formed in one of the shapesshown in FIGS. 4A, 4B, and 4C. Namely, in FIG. 4A, a tapered surface 41a is formed over the entire periphery around the outer peripheral edgeof a tip of the rotary shaft 41, while an inner peripheral surface 43 aof a central hole of the thrust plate 43 is adjacent to the taperedsurface 41 a. Accordingly, the relief portion 70 of a wedge-shaped crosssection is formed, and the two members are welded in this relief portion70. It should be noted that the tapered surface 41 a at the tip of therotary shaft 41 also functions as a guide portion at the time the thrustplate 43 is press-fitted to the rotary shaft 41.

FIG. 4B shows an example in which the tapered surface 41 a is formedover the entire periphery around the outer peripheral edge of the tip ofthe rotary shaft 41, while a tapered surface 43 b is also formed aroundthe inner peripheral edge of the central hole of the thrust plate 43.The two members are welded together in this relief portion 70.

In FIG. 4C, the tapered surface 41 a is formed over the entire peripheryaround the outer peripheral edge of the tip of the rotary shaft 41,while a flat recess 43 c is formed around the central hole at a bottomsurface portion of the thrust plate 43, a tapered surface 43 d beingformed around its outer periphery. Further, the hydrodynamic surface SBbis formed on its outer side. In the case of this example, a trapezoidalrelief portion 70 is formed, and the two members are welded together inthis relief portion 70.

Each of the relief portions 30 formed at the joining interface betweenthe rotary shaft 41 and the thrust plate 43 is formed at a positionoffset from the region where the dynamic pressure generating grooves areformed in the thrust plate 43. Accordingly, the dynamic pressuregenerating grooves are not subjected to limitations by the reliefportion 70, and it is possible to allow desired thrust hydrodynamicpressure to be demonstrated.

Further, as for the welding position, the entire periphery may bewelded, or welding may be effected partially at a plurality oflocations, insofar as the welding position or positions are located inthe relief portion 70.

As the welding process, it is possible to adopt a plasma weldingprocess, an arc welding process such as TIG welding, an electron beamwelding process typified by laser welding, or the like. In thisembodiment, the laser welding process is adopted in Which the basicmaterials to be joined are welded together by fusing the two materials.In this laser welding process, a laser beam emitted from a laseroscillator is focused by using a plurality of mirrors, and is radiatedto the joining interface to join the two members. According to such anelectron beam welding process, since a welding rod used in the arcwelding process is made unnecessary, the buildup of the basic materialin the joined interface portions can be minimized. Further, even if aslight buildup has occurred, since the axially recessed relief portion70 is provided at the joining interface, the built-up portion isaccommodated in the relief portion 70, and can be prevented fromprojecting from the hydrodynamic surface toward the counter plate 44side (see FIG. 2). Accordingly, it is desirable to set the size of therelief portion 70 by taking the size of the built-up portion intoconsideration.

If the arrangement is provided such that the built-up portion isaccommodated in the relief portion 70, the built-up portion is preventedfrom being located excessively close to the counter plate 44, and whenthe rotor assembly 40 including the thrust plate 43 is rotated, it ispossible to prevent the built-up portion from colliding against thebearing surface of the counter plate 44. Further, although the joinedportions of the rotary shaft 41 and the thrust plate 43 are located inthe lubricating fluid 5, since the two members are joined by weldingwithout using an organic solvent such as an adhesive agent, thecatalytic action with respect to the lubricating fluid 5 does not occur,so that the characteristics of the lubricating fluid 5 such as oil donot deteriorate.

Next, a description will be given of the structure for joining thebearing sleeve 33 and the counter plate 44 of the spindle motor shown inFIG. 2.

The disk-shaped counter plate 44 is secured in the opening at the lowerend of the bearing sleeve 33 formed in a hollow cylindrical shape. Thecounter plate 44 has its outer peripheral surface press-fitted to thebearing sleeve 33 with an appropriate press-fitting force, and an outerperipheral edge of its upper end face abuts against a stepped portion 33b of the bearing sleeve 33. Further, an axially recessed relief portion60 is formed in the portions of the obverse (lower) sides of the joininginterface portions of the bearing sleeve 33 and the counter plate 44,and the two members are integrated by welding in the relief portion 60.As the welding process, in the same way as the above-described processof joining the rotary shaft 41 and the thrust plate 43, it is possibleto use an electron beam welding process typified by laser welding.Accordingly, at least one of the bearing sleeve 33 and the counter plate44 is fused by being irradiated with an electron beam, thereby joiningthe two members.

Further, the shape of the relief portion 60 may be wedge-shaped,triangular, trapezoidal, or other cross-sectional shapes in the same wayas the shape of the stepped surface of the relief portion 70 formed atthe joining interface between the rotary shaft 41 and the thrust plate43 shown in FIGS. 4A to 4C. It should be noted that a tapered guideportion 33 c should preferably be formed at an inner peripheral edge ofthe opening of the bearing sleeve 33 so as to facilitate thepress-fitting or insertion of the counter plate 44. Further, as for thewelding position, it is preferable to weld the entire periphery so as toseal the opening.

In the structure for joining the bearing sleeve 33 and the counter plate44, the relief portion 60 is provided which is capable of accommodatingthe built-up portion formed by joining the joining interface portions,and welding is effected in this relief portion 60 to integrate the twomembers, as described above. Therefore, even if the built-up portion isformed by joining, the attempt to make the overall motor thin is nothampered. Furthermore, since the bearing sleeve 33 and the counter plate44 are joined by welding, it is possible to reliably prevent the leakageof the lubricating fluid 5 without using an O-ring or an adhesive agent.

Next, a detailed description will be given of the structure for joiningthe rotary shaft 41 and the hub 42 of the spindle motor in accordancewith this embodiment. As shown in FIG. 2, the joining length of therotary shaft 41 and the hub 42 is longer than the joining length of therotary shaft 41 and the thrust plate 43, but if the overall height ofthe motor is shortened, the joining length of the rotary shaft 41 andthe hub 42 also inevitably becomes short. Consequently, since thejoining strength of the rotary shaft 41 and the hub 42 declines.Accordingly, in this embodiment, in the same way as the structure forjoining the rotary shaft 41 and the thrust plate 43, the two members arejoined by welding after the rotary shaft 41 and the hub 42 arepress-fitted with an appropriate press-fitting force.

Here, if press-fitting is effected by providing a large press-fittingallowance of the hub 42 with respect to the rotary shaft 41, distortionoccurs in the hub 42 due to the press-fitting stress. Consequently, theperpendicularity of the hub 42 with respect to the rotary shaft 41,specifically the perpendicularity of the disk-mounting surface 42 b ofthe hub 42 with respect to the rotary shaft 41, becomes deteriorated, sothat the problem of occurrence of runout exceeding an allowable range isliable to occur when the disk is mounted on the hub 41 and is rotativelydriven.

Accordingly, in this embodiment, an axially recessed relief portion 50is formed at the joining interface between the rotary shaft 41 and thehub 42, and the two members are joined by laser welding in this reliefportion 50. The relief portion 50 is formed by a tapered surface 41 bformed at a corner of the tip of the rotary shaft 41 and a taperedsurface 42 c formed at an inner peripheral edge of a shaft-attachinghole 28 of the hub 42. Of these tapered surfaces, the tapered surface 41b of the rotary shaft 41 also functions as a guide portion at the timeof press-fitting the hub 42 to the rotary shaft 41. It should be notedthat, in this embodiment, since a damper guide 29 for guiding a damper(not shown) for holding the disk is provided on an upper end face of thehub 42 in such a manner as to axially project slightly from the joininginterface between the rotary shaft 41 and the hub 42, the attempt tomake the motor thin is not hampered even if the relief portion 50 is notformed. Further, as for the welding position, the entire periphery ofthe joining interface may be welded, or welding may be effectedpartially at a plurality of locations.

By virtue of the above-described joining structure, since the joiningstrength of the rotary shaft 41 and the hub 42 can be sufficientlyincreased without forcibly press-fitting the rotary shaft 41 and the hub42, the shock resistance of the motor improves, and the perpendicularityof the disk mounting surface 42 b of the hub 42 with respect to therotary shaft 41 can be maintained with high accuracy.

FIG. 5 is a half cross-sectional view showing a spindle motor inaccordance with a fourth embodiment of the invention. In FIG. 5, thosearrangements having common functions to those of the spindle motor shownin FIG. 2 are denoted by the same reference numerals, and a detaileddescription thereof will be omitted.

The stator cores 34 each having the coil 35 wound therearound areattached to the outer periphery of a tubular holder 32′ provideduprightly in the center of the fixing frame 31. This tubular holder 32′is formed to be axially longer than the tubular holder 32 shown in FIG.2, and the bearing sleeve 33 and the counter plate 44 are fixed to itsinner periphery. Namely, although the counter plate 44 in FIG. 2 isjoined to the opening of the bearing sleeve 33, in FIG. 5, the counterplate 44 is joined to the opening of the tubular holder 32′ of thefixing frame 31 after being press-fitted thereto with an appropriatepress-fitting force.

In joining the counter plate 44 to the tubular holder 32′, the axiallyrecessed relief portion 60 is provided at the joining interface betweenthe two members, and the counter plate 44 and the tubular holder 32′ arewelded in this relief portion 60 to integrate the two members. As thewelding process, the arc welding process or the electron beam weldingprocess is adopted as described above. Preferably, however, at least oneof the counter plate 44 and the tubular holder 32′ is fused by theelectron beam welding process typified by laser welding so as to jointhe two members. By joining the two members in the relief portion 60 inthis manner, since a portion projecting from the bottom surface of thefixing frame 31 or the counter plate 44 is not formed, the attempt tomake the motor thin is not hampered. Further, since the fixed shaft 31and the counter plate 44 are firmly joined by welding, the shockresistance also improves.

In this embodiment as well, the rotary shaft 41 and the thrust plate 43are joined in the same way as in the above-described embodiments.Namely, one end of the rotary shaft 41 is press-fitted in the centralhole of the thrust plate 43, the relief portion 70 is formed at thejoining interface between the rotary shaft 41 and the thrust plate 43,and the two members are integrated by welding in the relief portion 70.

Further, in the joining of the rotary shaft 41 and the hub 42, in thesame way as the joining of the rotary shaft 41 and the thrust plate 43,the rotary shaft 41 is press-fitted in the central hole of the hub 42,the relief portion 50 is formed at the joining interface between therotary shaft 41 and the hub 42, and the two members are integrated bywelding in the relief portion 50. Incidentally, this relief portion 50may be omitted depending on the shape of the hub 42.

As described above, in accordance with the spindle motor shown in FIG. 5as well, it is possible to obtain a sufficient joining strength even ifthe joining length of the rotary shaft 41 and the thrust plate 43 andthe joining length of the tubular holder 32′ of the fixing frame 31 andthe counter plate 44 are relatively short. Accordingly, it is possibleto stably maintain the perpendicularity of the thrust plate 43 withrespect to the rotary shaft 41. Moreover, even if projections are formedby welding, since the projections are respectively accommodated in therelief portions 60 and 70, the attempt to make the overall motor thin isnot hampered. Further, since the rotary shaft 41 and the thrust plate 43are joined by welding, even if the lubricating fluid 5 is oil, catalyticaction does not occur, and the characteristics of the lubricating fluid5 do not deteriorate.

Next, a description will be given of the structure for joining the fixedshaft 16 and the thrust plate 23 in FIG. 1. After the bearing sleeve 21formed integrally with the hub 22 is fitted over the fixed shaft 16provided uprightly on the fixing frame 11, the annular thrust plate 23is press-fitted to the fixed shaft 16 with an appropriate press-fittingforce. Subsequently, as the joining interface portions of the fixedshaft 16 and the thrust plate 23 are welded together, the two membersare joined. As shown in FIG. 6, the relief portion 70 which is recessedbelow the hydrodynamic surface is annularly formed at the peripheraledge of the central hole corresponding to the joining interface portionon the thrust plate 23 side. The laser welding process is desirable asthis welding, and the thrust plate 23 formed of a copper group material,a stainless steel metal, or the like is fused so as to undergo metallicfusion with the fixed shaft 16. The welding with the fixed shaft 16 isperformed in the relief portion 70, and the arrangement provided is suchthat even if a local projection occurs due to welding, it does notproject above the hydrodynamic surface.

By virtue of such an arrangement, since a sufficient joining strengthcan be obtained even if the joining length of the fixed shaft 16 and thethrust plate 23 is relatively short, the perpendicularity of the thrustplate 23 with respect to the fixed shaft 16 can be maintained stably, sothat the reliability of the motor improves. Moreover, since the axiallyrecessed relief portion 70 is provided at the joining interface, and thetwo members are integrated by welding in this relief portion 70, theattempt to make the overall motor thin is not hampered. Further, sincethe joining interface portions located in such a manner as to becontiguous to the lubricating fluid 5 for generating hydrodynamicpressure are welded, even if the lubricating fluid 5 is oil, catalyticaction does not occur, and the characteristics of the lubricating fluid5 do not deteriorate.

Although a description has been given above specifically of theembodiments of the invention devised by the present inventors, theinvention is not limited by the foregoing embodiments, and it goeswithout saying that various modifications are possible without departingfrom the scope of the invention.

For example, although, in the above-described embodiment, an example hasbeen shown in which joining is accomplished by welding in such a waythat the counter plate 44 closes the bearing sleeve 33 or the opening ofthe tubular holder 32′ of the fixing frame 31, part of the joininginterface may be welded to secure joining strength, and the entireperiphery of the joining interface may be sealed by an adhesive agent.Consequently, it is possible to reliably prevent the leakage of thelubricating fluid.

Furthermore, the invention is similarly applicable to a spindle motorthan a hard-disk driving motor, e.g., a CD-ROM driving motor and apolygon-mirror driving motor.

What is claimed is:
 1. A spindle motor comprising: a fixed shaft; acylindrical rotary bearing member rotatably supported on an outerperipheral face of the fixed shaft, and made of a first metal material;a rotary hub integrally joined to the rotary bearing member, and made ofa second metal material different from the first metal material; and apotential-difference alleviating member provided on the joining surfacesof the rotary bearing member and the rotary hub, and made of a thirdmetal material whose ionization tendency in an electrochemical series ispositioned between ionization tendencies of the first and second metalmaterials.
 2. The spindle motor as set forth in claim 1, wherein thefirst metal material is a copper group metal material, the second metalmaterial is an aluminum group metal material, and the third metalmaterial is a nickel group metal material.
 3. The spindle motor as setforth in claim 1, wherein the potential-difference alleviating member isformed on at least one of the joining surfaces of the rotary bearingmember and the rotary hub by any one of plating, vapor deposition andcoating.
 4. A spindle motor comprising: a fixed shaft; a cylindricalrotary bearing member rotatably supported on an outer peripheral face ofthe fixed shaft, and made of a first metal material; and a rotary hubintegrally joined to the rotary bearing member, and made of a secondmetal material different from the first metal material; and apassivation film formed on the joining surfaces of the rotary bearingmember and the rotary hub.
 5. The spindle motor as set forth in claim 4,wherein the passivation film is made of either the first metal materialor the second metal material.
 6. The spindle motor as set forth in claim5, wherein the passivation film is made of a third metal material whichis different from the first and second metal materials.
 7. A spindlemotor comprising: a fixed frame made of a first metal material; acylindrical fixed bearing member integrally joined to the fixed frame,and made of a second metal material different from the first metalmaterial; a rotary shaft rotatably supported on an inner peripheral faceof the fixed bearing member; a rotary hub secured to the rotary shaft;and a potential-difference alleviating member provided on the joiningsurfaces of the fixed frame and the fixed bearing member, and made of athird metal material whose ionization tendency in an electrochemicalseries is positioned between ionization tendencies of the first andsecond metal materials.
 8. The spindle motor as set forth in claim 7,wherein the first metal material is a copper group metal material, thesecond metal material is an aluminum group metal material, and the thirdmetal material is a nickel group metal material.
 9. The spindle motor asset forth in claim 7, wherein the potential-difference alleviatingmember is formed on at least one of the joining surfaces of the rotarybearing member and the rotary hub by any one of plating, vapordeposition and coating.
 10. A spindle motor comprising: a fixed framemade of a first metal material; a cylindrical fixed bearing memberintegrally joined to the fixed frame, and a second metal materialdifferent from the first metal material; a rotary shaft rotatablysupported on an inner peripheral face of the fixed bearing member; arotary hub secured to the rotary shaft; and a passivation film formed onthe joining surfaces of the fixed frame and the fixed bearing member.11. The spindle motor as set forth in claim 10, wherein the passivationfilm is made of either the first metal material or the second metalmaterial.
 12. The spindle motor as set forth in claim 11, wherein thepassivation film is made of a third metal material which is differentfrom the first and second metal materials.
 13. A spindle motorcomprising: a fixed shaft; a cylindrical rotary bearing member rotatablysupported on an outer peripheral face of the fixed shaft, and made of afirst metal material; a rotary hub integrally joined to the rotarybearing member, and made of a second metal material different from thefirst metal material; and an insulating resin film formed on the joiningsurfaces of the rotary bearing member and the rotary hub.
 14. Thespindle motor as set forth in claim 13, wherein the resin film is formedon outer circumferential faces of the rotary bearing member and therotary hub continuously from the joining surfaces such that the rotarybearing member and the rotary hub are partly conducted.
 15. A spindlemotor comprising: a fixed frame made of a first metal material; acylindrical fixed bearing member integrally joined to the fixed frame,and made of a second metal material different from the first metalmaterial; a rotary shaft rotatably supported on an inner peripheral faceof the fixed bearing member; a rotary hub secured to the rotary shaft;and an insulating resin film formed on the joining surfaces of therotary bearing member and the rotary hub.
 16. The spindle motor as setforth in claim 15, wherein the resin film is formed on outercircumferential faces of the rotary bearing member and the rotary hubcontinuously from the joining surfaces such that the rotary bearingmember and the rotary hub are partly conducted.